Evaluating the quality of a received signal is important for the development, installation, and maintenance of a wireless digital communication system. If the quality is degraded, it is necessary to evaluate both the air conditions of multi-propagation paths and the apparatuses, e.g., transmitters and receivers, in order to determine the cause. In the case of conventional Single-Input, Single-Output (SISO) communication, a transfer function or delay profile is measured to evaluate the propagation path.
Wireless Local Area Network (LAN) standard IEEE802.11n uses Space Division Multiplexing (SDM) and Multiple-Input, Multiple-Output (MIMO) to improve the transfer rate. MIMO communication transmits different data with a plurality of transmitting antennas and receives them with a plurality of receiving antennas. Compared to conventional SISO communication, the qualities of the received signals are significantly affected by the propagation paths from the transmitting antennas to receiving antennas. A receiver demodulates data using the transfer functions of the respective propagation paths. In the case of N×N MIMO, there are N×N transfer functions between the transmitting and receiving antennas, and demodulation of data from one transmitting antenna requires a calculation using all of the transfer functions. Therefore, it is difficult to decide whether the propagation paths are good or not by merely observing the transfer functions or delay profiles.
FIG. 1 depicts a conventional quality evaluation of a 2×2 MIMO signal—one data symbol of IEEE802.11n OFDM. Referring to FIG. 1A, marker 10 of in the graph “EVM vs SC” indicates that subcarrier (SC) number 7 has Error Vector Magnitude (EVM) of 72.968%—a very bad value of signal quality. Because the EVM reflects the conditions of both the propagation paths and the apparatuses, it is important to separately evaluate the conditions of the propagation paths and the apparatuses in order to determine the cause of the quality degradation. In the case of conventional SISO communication, the propagation path can be evaluated directly from the transfer function. For example, FIGS. 1B and 1C show transfer functions of amplitudes (“Amp”) and FIGS. 1D and 1E show phases (“Ph”) of four paths (Tx1-Rx1, Tx2-Rx1, Tx1-Rx2, and Tx2-Rx2). Markers 12-18 indicate positions corresponding to subcarrier number 7. Unfortunately, it is very difficult to determine if the propagation paths are the causes of the quality degradation because the graphs of the FIGS. 1B-1E do not show distinctive portions in the graph shapes at the marker positions.
What is desired is an apparatus and method for evaluating conditions of propagation paths of wireless communication systems having a plurality of transmitting and receiving antennas.